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FDP Forum / Guitar Mods, Repairs, and Projects / Piezo / Preamp Noise problem

wrnchbndr
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New Jersey

I'm back with the otters again
Feb 8th, 2019 09:58 AM   Edit   Profile  

Every once in a while I'll get in an acoustic guitar that has what presents itself as a ground hum problem. These could be any typical manufactured acoustic with an undersaddle piezo and active preamp. The symptoms are excessive noise at higher signal levels and noise interrupt when you touch the metal backshell of the instrument cable and, the pickup is still actually functioning with a strong output (its understood that the cable and the amplifier has been ruled out). Sometimes its just a dirty/defective or miswired output jack and that is my first thought. But I have had instances where the cause has been a defective undersaddle piezo pickup. The fact that there is what seems to be ground problem and the noise is impacted by touching a finger to the backshell of the instrument cable causes me to not be so confident.

For clarification,... There are a number of piezo formats ranging from the simple piezo cable which is just a thin coaxial wire with the insulator made of piezo material to a number of other sandwich styles. The ones that I've had problems with are the very plain rigid strip that looks like it has a thin covering of foil and a typical example is the Martin Thinline. The current replacement from Fishman is called the AG Series.

I have personally accidentally damaged a piezo while rushing during a saddle height adjustment resulting in this noise problem and have successfully addressed this problem on at least two other occasions by replacing the piezo. Over the years I've purchased quite a few piezo pickups on ebay at dirt cheap prices in lots with a single unit cost of about $2 and have accumulated a wide range of sizes and thicknesses. The $2 piezos work great but it is rare to find them on ebay in the narrow width format (3/32 / 2.3mm) leading to the necessity of paying around $100 for a narrow format pickup from Fishman. Hey, if the client has a defective pickup and the cost is $100, that's okay but I don't want to spend 100 dollars of the client's money on a guess. Talking to Fishman support, they concur that the problem is likely a defective pickup but I feel so much more confident when I troubleshoot a problem and discover the cause and prove it through an actual measurement or visual confirmation. Swaptronics is a proven technique but I don't like it when its expensive.

The first thing is that I don't understand the theory or reason a piezo element would do this. My guess is a hairline fracture of the foil-like outer conductor. This foil is delicate and easily torn by sharp objects. I understand that this foil is the equivalent of the ground side of a magnetic pickup.

Can I replace the pickup with a 1 Meg resistor at the preamp and if the noise problem at high gain goes away be confident that the problem is actually the piezo? Any more thoughts on this are appreciated.

Peegoo
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Just beyond Mars

there's a world of fools
Feb 8th, 2019 11:04 AM   Edit   Profile  

I've dealt with noisy under-saddle piezos before. If the noise stops when you touch the barrel of the plug, it's almost always a shielding issue. I approach this problem in steps, because one of the preliminary steps might solve the issue.

Inspect the piezo element to ensure the copper foil is intact, undamaged, and in good contact with the piezo strip. Any scratches, rips, or bubbles will create noise. If the transducer is good, next step:

Inspect the wiring to ensure all ground shielding is good. It's common to have an unshielded signal wire at the solder tab on the output jack. You can put some shrink tubing over it, followed by a wrap of copper foil with a soldered connection to ground.

If none of that solves the noise problem, I add a string ground. It is dead simple on an acoustic guitar.

Cut a strip of adhesive copper foil 3/4" wide and 3" long. On one end, solder a 12" length of thin (20-ga, etc.) wire.

Loosen the guitar strings to really slack, and place a capo on the 1st fret. Pull the six string pins and remove the strings from the bridge. Peel the backing off the copper tape and place it against the bridge plate inside the guitar, centered over the sting holes. Place the tape so the soldered wire is on the treble side.

From the inside of the guitar, use a golf tee to poke six holes through the tape. Press the tee upward as far as it will go so the small portions of broken tape will be firmly pressed against the sides of the string holes in the bridge. A small Phillips screwdriver will work, but the tee is way better because it has a taper and does a better job of pressing the foil into good contact. If you don't have a golf tee, use a bridge pin.

Remove the output jack. Feed the string ground's wire out through the output jack mounting hole and solder it to the shield side of the jack.

For tail block-mounted barrel jacks, run the string ground wire through the jack's mounting nut and washer before you bring it through the hole in the guitar, or you'll be unsoldering the wire :^o

Reinstall the jack. Done.



wrnchbndr
Contributing Member
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New Jersey

I'm back with the otters again
Feb 8th, 2019 11:10 AM   Edit   Profile  

This has worked for you?

Peegoo
Contributing Member
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Just beyond Mars

there's a world of fools
Feb 8th, 2019 12:36 PM   Edit   Profile  

Yep. But sometimes no matter what I do, the noise persists.

I wonder if the piezo wafers in some of these these things crack over time. I would guess that is the cause of the noise that persists even touching a ground contact..

(This message was last edited by Peegoo at 02:38 PM, Feb 8th, 2019)

FDP Forum / Guitar Mods, Repairs, and Projects / Piezo / Preamp Noise problem




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